The situation is you have been dragged into an issue whereby a fraudster has stolen a mobile phone from VM.
You have done your best to assist in the matter even to the extent of contacting the police (I am sure that this would once have been taken on board to do some enquiries but I am not surprised at whats happened - you are reporting being incorrectly billed? - its a matter for VM to pursue if they so wish).
Unfortunately the response you have received to your help and willingness to assist and cooperate is to have been probably automatically put on a debt collectors list and this, as Gorf points out, could be the real problem for you.
A reputable debt collection agency (they do exist) will immediately cease contacting you if they know the 'debt' is in dispute - this is the law and they should be less than impressed by VM. There is no harm done if you contact the debt collector and explain the situation. Also contact VM/complain and demand to know what they are doing about it, the more times the better as you should be keeping any documentation and a log of all contacts and conversations with names and times recorded.
Once you can show VM are aware of the situation any further requests for money from you most certainly is a serious criminal matter (demanding with menaces/blackmail).
While it is always better to resolve issues in a reasonable way I would lean towards getting professional legal assistance from a solicitor in these circumstances or at least contacting the Citizens Advice Bureau who are free and usually very good.
Of course the best course would be for a moderator to bring this thread to the notice of the Fraud Department and for you to be contacted for your kind assistance (an apology would also not be out of place) and for this nonsense to be sorted out.
Please note I am taking your post at face value - if you were involved in a scam where you end up paying bills I don't think you'd put it on a public forum or go to the police.
Good luck with it - an update when available would be of interest should you care to do so.